Insufficient blood oxygenation, known as hypoxemia, can cause irreversible injury or even death. For example, surgical patients are vulnerable to hypoxemia during anesthesia. Similarly, hypoxemia may occur during recovery from anesthesia, during critical care treatment (also known as intensive care), and at other times when patient airway functions or cardiopulmonary functions may be compromised during periods of medical care on hospital general medical/surgical wards or during home care. A patient may be particularly vulnerable when dependent on supplemental oxygen or an artificial airway. Early warnings of hypoxemia, in these and other situations if adequately provided, can permit clinicians sufficient opportunity to intervene and prevent occurrence of irreversible injury. Examples of monitoring equipment that have been used to provide warnings of the onset of hypoxemia include non-invasive multi-wavelength spectrometers, such as pulse oximeters. Pulse oximeters are used by anesthesiologists, surgeons, critical care physicians, emergency medical physicians and other clinicians, including home care providers. To effectively provide warning though the equipment must be capable of continuous, accurate and real time measurement of patient hemoglobin oxygen saturation.
Since the mid-1930s it has been known that attenuation measurements of light passed through blood, either in vivo or in vitro, can be used to determine hemoglobin oxygen saturation, i.e., blood oxygenation concentrations. Technologies for such measurements rely on the fact that hemoglobin in blood can be loosely combined with oxygen in the form of oxyhemoglobin for transport to various body tissues where oxygen can be released. This chemistry supports in vivo optical measurements of blood oxygenation concentrations because light extinction factors, i.e., the magnitudes of attenuation, for oxyhemoglobin are different from that for hemoglobin. For example, hemoglobin transmits much less visible red light (620-770 nanometers (nm)) than does oxyhemoglobin. Therefore, blood with high oxygen concentrations will transmit more visible red light than will blood with low oxygen concentrations. On the basis of these facts oximeter instruments using pulsed light sources in combination with photosensors to measure light intensities transmitted through patient tissue have been developed for determining in vivo blood oxygenation concentrations. In general such oximeter instruments include a photoelectric probe and an electronic processor. Typically, the photoelectric probes, which include light sources and photosensors, are positioned on a patient so light can be directed to pass through tissue, i.e., forward-scattered, before being received by photosensors. Convenient locations for mounting these photoelectric probes on patients include fingers and ears. Alternative photoelectric probes rely on back-scattering to effect light attenuation for determining blood constituent concentrations. Electronic processors for oximeter instruments are used in conjunction with photoelectric probes of either type for controlling power to light sources, measuring photosensor detected light signal waveform amplitudes, determining attenuation of light passed into patient tissue, and providing read outs of blood oxygenation concentration levels determined from identified attenuation magnitudes. A pulse oximeter of this general type is disclosed in U.S. Pat. No. 4,621,643 to New, Jr., et al.
Today, pulse oximeters are virtually standard equipment in hospital operating rooms and other facilities, such as intensive care units, where patients require real time accurate in vivo monitoring of blood oxygenation concentration levels. In fact, there is now a recognized and accepted critical requirement for real time accurate in vivo monitoring of blood oxygenation concentration levels. Specifically, the need and the commensurate capability provided by pulse oximeters resulted in a 1986 issuance of standards recommending use of pulse oximeters by the American Society of Anesthesiologists. Accordingly use of pulse oximeter equipment is rapidly expanding into hospital general medical/surgical wards and is also developing an acceptance as a requirement for home care.
To be effective oximeter electronic processors must be as fully automated as possible for unattended operation over extended periods of time so there is provision of as near real time continuous accurate blood oxygenation concentration measurements as possible. These requirements are integral with situations where such instruments are needed. For example, during surgery anesthesiologists and other physicians need current accurate in vivo information on patient blood oxygenation concentration levels over extended periods of time and this information must be made available with minimum to preferably no requirements for manual adjustment of oximeter equipment. With such automated capabilities for real time, continuous, accurate measurements, physicians and other medical personnel can have essential blood chemistry information while attending to other tasks required for patient care.
Providing automated operation of oximeter equipment requires use of calculating and control circuitry integrally provided by central processor units (CPU). The CPU in such equipment is not only used for commanding display of determined blood oxygenation concentration levels and calculating these levels using measured light intensities, but also for adjusting: light intensity levels prior to transmission into patient tissue; circuitry gains for measuring light intensities; and, can even be required for adjusting rates for sample-and-hold (s/h) circuits and analog-to-digital (a/d) converters. All tasks associated with automation consume time and CPU calculating capacity. In fact, electronic circuitry in current oximeter electronic processors requiring automated monitoring and adjustment necessitate extensive use of hardware and software accordingly reducing the amount of processor time and capacity available for processing measured light signal waveform levels and providing improved oximeter accuracy. Consequently, use of CPU capacity for real time performance and expanded calculations for achieving ultimate accuracy must be traded off against functions required for automated operation.
Critical to both consumption of CPU capacity and instrument accuracy are s/h and a/d circuits used in oximeter electronic processors. In order to maximize accuracy, these circuits can require both adjustment of their sample rates to optimize digitizing of measured light signal waveforms for CPU processing, and also adjustment of associated circuitry gains, to include drive circuits for controlling generated light source intensities. Adjustment of light source intensity can be used to assist s/h and a/d circuits in covering dynamic ranges consumed by measured light signal waveform amplitudes. This later aspect of being able to cover measured light signal waveform dynamic ranges has a direct effect on accuracy in determining blood oxygenation concentration levels. In particular, s/h circuits available at reasonable cost do not have sufficient capacity to adequately cover dynamic ranges needed for measuring light intensity signal waveform amplitudes without implementing automated adjustment of currently used associated circuitry gains. Even with such automated adjustment, however, current oximeter electronic processors must trade off accuracy against required dynamic range coverage because of s/h limitations.